Optical Lever Measurement Accuracy for Off-Resonance Atomic Force Microscopy

Abstract

This research evaluates measurement accuracy in optical lever-based atomic force microscopy (AFM) for off-resonance conditions and parameter variations. Under controlled conditions and correct calibration, AFM provides researchers with the ability to accurately observe and manipulate matter on the micro- and nano-scale. Accuracy of imaging and nano-manipulation operations are directly correlated to the accuracy with which the displacement of the probe is measured. The optical lever method, a common displacement measurement technique employed in AFM, calculates probe displacement based on a calibration that assumes a consistent response profile throughout operation. Off-resonance excitation and tip-sample interaction forces during intermittent contact mode AFM can alter this response profile. Standard tapping-mode operation at the fundamental frequency is observed to be robust to changes in effective stiffness, maintaining accurate measurements for all laser spot positions considered. A nominal laser spot position between Xp = 0.5 and 0.6 is determined to most accurately predict displacement for off-resonance excitation during both free response and intermittent contact condit ions. Measurement accuracy for off-resonance tapping- mode is more directly correlated to changes introduced to the interaction force profile than choice of spot position.